Telecom Glossary

3GPP
Third Generation Partnership Project - A body comprising several organizational partners working to produce technical specifications for a third- generation mobile system based on GSM core networks and the radio technology they support.

ACF
ADMISSIONS CONFIRM message - A RAS message that the Gatekeeper sends to the calling point, accepting the ARQ.

Address Resolution
A mechanism for identifying the address of a called endpoint in terms of the network, such as an IP address.

Address Translation
The ability of a Gatekeeper to translate an alias address, such as a name or e-mail address, to a transport address. One method of translation uses a Translation Table, which is updated by the Registration messages on the RAS channel.

Alias
An alternative identification string for an IP address. An alias can be a name, a URL address, an e-mail address, a transport address in the form of "IP address port number", or a Party Number.

Algorithm
1. Rule of thumb for doing something with a semblance of intelligence. For example, a descrambling algorithm will yield a clear, unscrambled message from an apparently meaningless one. 2. The procedure used for performing a task.

Alternate Gatekeeper
Support for an Alternate Gatekeeper enables you to make Gatekeeper failures transparent to the endpoints that are registered to the Gatekeeper. In RADVISION implementations, a backup Gatekeeper (the "Secondary" Gatekeeper) runs in parallel to each online Gatekeeper (the "Primary" Gatekeeper).

Analog
Information represented by a continuous electromagnetic wave encoded so that its power varies continuously with the power of a signal received from a sound or light source.

ANI
Automatic Number Identification. The automatic identification of a calling station, usually for automatic message accounting. Also used in pay-per-view automated telephone order entry to identify a customer for billing and program authorization purposes.

Application Level Gateway
Application Level Gateways (ALGs) serve as communicators between two networks. ALGs are protocol-aware entities that examine application protocol flows and only allow messages that conform to security policies to pass. See also proxy server.

ARJ
ADMISSIONS REJECT Message - A RAS message that the Gatekeeper sends to the calling point, rejecting the ARQ.

ARQ
ADMISSIONS REQUEST Message - A RAS message send by an endpoint placing a call or an endpoint receiving a call asking for bandwidth allowance and permission to continue the Call Setup.

ATM
Asynchronous Transfer Mode. A high bandwidth, controlled-delay, fixed size packet switching and transmission system. Uses fixed size packets, also known as "cells"; ATM is often referred to as "cell relay". ATM will provide the basis for the future broadband ISDN standards.

Authentication
The process of verifying the identity of a user trying to log on to a system, or of the sender of a message.

Bandwidth
Determines the rate at which information can be transmitted across a medium. The rates are measured in bits (b/s), kilobits (Kb/s), megabits (Mb/s) or gigabits per second (Gb/s). Typical transmission services are 56Kb/s, 64Kb/s, 1.544Mb/s (T1) and 45Mb/s (T3).

Bearer Channel
Term used to define a channel that carries voice, data or video information.

Bit
Contraction of the term Binary digiT. The smallest unit of information a computer can process, representing one of two states (usually indicated by "1" or "0").

BISDN
Broadband ISDN. In 1995-1996, BISDN began to offer dedicated circuits, switched circuits and packet services at rates of 155Mb/s and above. BISDN is still relatively in the conceptual stage. The goal is to take advantage of the raw bandwidth, which has been made available by the proliferation of fiber optic cable plants.

Blanking Level
The level of the front and back porches of the composite video signal.

Bonding
Method for making several BRI lines look like one high-rate line by use of an IMUX (inverse multiplexer).

bps
Bits per second - A unit of measurement of the speed of data transmission and thus of bandwidth (lower case is significant).

Bps (or BPS)
Bytes (8-bit) bytes per second. A unit of measurement of the speed of data transmission and thus of bandwidth (upper case is significant).

BRI
Basic Rate Interface. An ISDN subscriber line, consisting of two 64Kb/s B channels (bearer channels) and one 16Kb/s D channel (used for signaling and synchronization purposes.) - often referred to as 2 B's and a D.

Bridge
An interconnection device that can connect LANs using similar or dissimilar media and signaling systems such as Ethernet, Token Ring and X.25. A bridge is also called a data link relay or level 2 relay. Connects remote sites over dedicated or switched lines to create WANs. Also the device that allows multiple locations (more than 2) to videoconference simultaneously.

Broadcast
Transmission of data to everybody on the network or network segment.

Buffer
See jitter buffer.

Byte
A group of bits treated as a unit used to represent a character in some coding systems. Typically, eight bits equals a byte.

Call Acceptance
Acceptance or rejection of calls from an H.323 terminal. The Gatekeeper may reject calls from a terminal because of restricted access to or from particular terminals or Gateways, or restricted access during certain periods of time. Call Authorization is an optional Gatekeeper service.

Call Control
Also called Call Processing. Refers to the signaling involved in setting up, monitoring, transferring, and disconnecting (tearing down) a call.

Call Feedback
Support for Call Feedback enables you to configure a Gatekeeper Forwarding policy to deal with cases such as Gatekeeper failure to resolve a destination address in IP network, lack of Gatekeeper bandwidth resources, or unsuccessful Call Setup to the destination endpoint due to network failure.

Carrier
Vendor of transmission services operating under terms defined by the FCC as a common carrier. Owns a transmission medium and rents, leases or sells portions for a set tariff to the public via shared circuits. (AT&T, Sprint, MCI, Ameritech, etc.)

Cascaded MCU
The MCU allows you to combine two or more conferences resulting in a larger conference with many more participants. This is called Cascading. Cascading creates a distributed environment that helps reduce the drain on network resources.

CCITT
Consultative Committee on International Telephony and Telegraphy. An international standards group.

CDR
Call Detail Record - Information in a simple text format that can be used as input to third party billing programs or other software for billing purposes.

Channel
A signal path of specified bandwidth for conveying information such as voice, data and video.

Chat Room
A virtual room where a chat session takes place. Technically, a chat room is really a channel, but the term room is used to promote the chat metaphor.

Chip Sets
Application-specific integrated circuits (ASICS) are being developed for use in video application products such as codecs, desktop video and home satellite entertainment. ASICS operate more like computer hardware. Programmable chips operate much like computer software. The chip sets meet the CCITT H.261 compression standard and will be the driving force in the widespread use of video communications technology because they will lower the cost and open up the technology to a much larger group of users.

CIF
Common Intermediate Format. The CCITT standard that addresses the incompatibility between the European television standard PAL (Phase Alternation Line) and SECAM (Systeme Electronique pour Couleur Avec Memoire) and those in most areas of the rest of the world that utilize NTSC (National Television System Committee). In the encoding process, CIF is divided into 12 GOBs (Groups of Blocks).

Circuit
1. Means of two-way communication between two or more points. 2. In communications systems, an electronic, electrical or electromagnetic path between two or more points capable of providing a number of channels.

Circuit Switching
A networking technology that provides a temporary but dedicated connection between two stations regardless of the number of switching devices through which data is routed. Analog circuit switching (FDM) has been replaced by digital circuit switching (TDM). The digital technology still maintains the connection until one speaker hangs up.

Cisco Proxy
The Cisco H.323 Proxy is a device that acts like a Gateway and relays H.323 data between H.323 zones.

Clustered MCUS
The Multipoint Controller (MC) and Media Processor (MP) unit components of the MCU operate independently. The MCU can be set up in a clustered layout to use a single MCU to control several units configured to operate only as MP units performing media processing. MCUs configured as MP Only units have their MC component disabled. The controlling MCU unit also makes use of the local MP component.

Codec
COder-DECoder. A video codec converts the analog video signals from a video source to digital signals for transmission over digital circuits, then converts the digital signals back to analog signals for display. An audio codec converts the audio signals to digital signals for transmission over digital circuits, then converts the digital signal back to analog for reproduction.

Common Carrier
Usually a telecommunications company that owns a transmission medium and rents, leases or sells portions for a set tariff to the general public via shared circuits through published and nondiscriminatory rates. (MCI, etc.)

Compression
The method of taking raw data and processing it so that it may be represented with less information (or bits in the digital world.) Compression falls into two categories: lossless - the original data may be completely recovered - and lossy - the representation of the original data contains errors.

Compressed Video
Processed video images; transmits changes from one frame to the next which reduces the bandwidth to send them over a telecommunications channel which reduces cost. Also called bandwidth compression or bit rate reduction.

Conference Hunting
The purpose of Conference Hunting in RADVISION implementations is to maintain conferences and ignore Line Hunting where necessary.

Conferencing Service
The Conferencing Service is a mechanism supported on the RADVISION MCU which defines the qualities, compatibilities and management policies of a conference.

Continuous Presence
Continuous Presence (CP) allows you to view multiple participants in one screen at the same time. Incoming participant images are combined into a video image layout set according to the policies of the conferencing service. The range of video layouts available depends on the type of media processing supported.

CPE
Customer Premise Equipment. Terminal equipment located on the customer premises which connects to the telephone network.

CSU
Channel Service Unit. A device used to connect a digital phone line coming in from a carrier to network access equipment located on the customer premises. A CSU may also be built into the network interface of the network access equipment.

D Channel
The data signaling channel of an ISDN line. This channel is used to carry call control messages between the ISDN terminal and the public switch.

DCT
Discrete Cosine Transform. A compression algorithm.

Default Extension
Any endpoint on the network that has been defined as an alternative destination for incoming calls routed by a Gateway.

Definition
Also called resolution. The fidelity with which detail is reproduced by a television or video display system ranging from fuzzy to sharp appearance.

Delay
The time taken for a signal to pass through a videoconference from the sending station to the receiving station.

DES
Data Encryption Standard. An algorithm for encrypting (coding) data designed by the National Bureau of Standards so it is impossible for anyone without the decryption key to get the data back in unscrambled form.

Desktop Videoconferencing
Videoconferencing on a personal computer - Most appropriate for small groups or individuals. Many desktop videoconferencing systems support document sharing.

DHCP
Dynamic Host Configuration Protocol - In a DHCP environment, IP policy is dynamic. This means that a terminal does not have a constant IP address. Management keys for identifying endpoints in a DHCP environment are the alias name or phone number of an endpoint.

Dial Plan
In traditional telephony systems, a dial plan is a front end system that allows users to call each other by dialing a number on a telephone. In voice and videoconferencing over IP, a dial plan is a system that allows participants in point-to-point or multipoint conferences to call each other or join conferences. The RADVISION ECS Dial Plan provides "configuration tools" which allow network administrators to build am IP dial plan that suits the requirements of their organization and network.

DID
Direct Inward Dialing - A method in which incoming calls are routed directly to endpoints on the LAN, without operator intervention.

Digital
Discrete bits of information in numerical steps. A form of information that is represented by signals encoded as a series of discrete numbers, intervals or steps, as contrasted to continuous or analog circuits.

Digital Dial-Up Bandwidth
Communications channels created by signaling to the network from the caller's site the intended destination of the connection. These channels may be terminated when the caller or called party chooses. The user pays for the bandwidth only when it is used. Digital Dial-Up Bandwidth operates in a fashion similar to the dialed voice telephone network, but the resultant connections are digital and of specified bandwidth.

Digital Media
Refers to any type of information in digital format including computer-generated text, graphics and animations, as well as photographs, animation, sound and video.

Direct Mode
Routing of the Call Setup channel (Q.931) and the control channel (H.245) to form a direct connection between two endpoints without Gatekeeper intervention. See also Call Setup routing.

Directional Microphone
A microphone that detects and transmits sound from only a certain direction. Useful in preventing unwanted sound from being transmitted.

Display
The visual presentation on the indicating device of an instrument.

Distance Learning
Incorporation of video and audio technologies so that students can "attend" classes and training sessions presented at a remote location.

DNS
Domain Name Server - On TCP/IP networks, DNS converts the domain name (URL) of a host computer into a numeric IP address using the following format xxx.xxx.xxx.xxx.

Document Sharing
A videoconferencing feature that enables multiple participants to view and edit the same computer document.

Double-Talk
The situation where parties at both ends of a conference are speaking simultaneously. A quality echo canceller will provide a continuous speech path I-directionally during double-talk.

Downspeeding
The ability of a Gateway to maintain a ISDN video call by reconnecting the call at a lower rate when one of the B channels is lost. Downspeeding contributes to a higher percentage of call completion on the network.

DSO
One 64 Kb/s unit of transmission bandwidth. A worldwide standard for digitizing one voice conversation and more recently for data transmission. Twenty four DSO's equal one DS1 (64Kb/s x 24) or a T-1.

DS1
One 544Mb/s unit of transmission bandwidth in North America or one 2.048Mb/s unit of transmission bandwidth elsewhere in the world. A telephony term describing a 1.544 or 2.048Mb/s digital signal carried on a T1 circuit.

DTMF
Dual Tine Multi-Frequency signals - Also called touch-tone dialing. The tones that are heard when you press the buttons on a touch-tone telephone.

Dual 56
Two switched 56 calls made between videoconferencing equipment to allow data transfer at 112Kb/s. The videoconferencing equipment performs a two channel inverse multiplexing procedure to assure channel alignment.

DVI
Digital Video Interactive. DVI is a programmable (variable bit and frame rate) compression / decompression technology developed by Intel offering two distinct levels and qualities of compression / decompression for motion video. Production Level Video (PLV) and Real Time Video (RTV) use variable compression rates. PLV is a proprietary compression technique that is well suited for encoding full motion. PLV emulates MPEG and has a very high image quality. RTV provides comparable image quality to frame rate (motion) JPEG and uses a symmetrical variable rate compression.

Dynamic Bandwidth Allocation
The process of determining current traffic loads over a channel and automatically increasing or decreasing the bandwidth of the channel to optimize the utilization of bandwidth efficiency

E1:2.048Mb/s.
Common European equivalent of a T-1.

E.164
The term "E.164 number" differentiates an "absolute" telephone number from the number you must dial to reach an endpoint form a specific location. E.164 numbers include country codes, national destination codes and subscriber numbers.

E-box
Electronics box (the main codec box) of a group videoconferencing system.

Echo Canceller
Eliminates audio transmission echo. A telephone line echo canceller produces a synthetic replica of the echo it expects to see returning and subtracts it from the transmitted speech. The replica it creates is based on the transmission characteristics of the telephone cable between echo canceller and the telephone or videoconferencing system. Device that allows for the isolation and filtering of unwanted signal caused by echoes from the main transmitted signal.

Echo Effect
A time-delayed electronic reflection of a speaker's voice. This is largely eliminated by modern digital echo cancellation.

Echo Suppression
A technique for reducing annoying echoes in the audio portion of a videoconference by temporarily deadening the communication link in one direction.

Electronic Blackboard
A device or whiteboard that looks like an ordinary blackboard or whiteboard, but has a special conductive surface for producing free hand information that can be sent over telephone lines.

Endpoint
A network element at the end of the network such as an H.323 terminal, a Gateway, a Multipoint Controller Unit (MCU), a PC terminal, IP or ISDN phone, or video conference.

Ethernet
A LAN physical and data link protocol running over the lowest two layers of the OSI Reference Model at speeds of up to 10 or 100 Mbps.

ETSI
European Telecommunications Standards Institute. A France-based non-profit making organization that produces telecommunications standards to be used throughout Europe and beyond.

Exit Zone
When you define a prefix for the Exit Zone Service, you need to dial the prefix to reach an endpoint in another zone. This can be useful for restricting unauthorized users from making calls to other zones. The Exit Zone prefix affects the way in which the Gatekeeper tries to complete calls to other zones.

Fallback
A modem speed that is lower than its normal (that is, maximum) speed of operation.

Fast Connect
Also called Fast Start and H.323 Fast Start. A procedure for shortening the time it takes to start a call by skipping the H.245 phase and transferring channel parameters in the Call Setup messages.

Fast Start
See Fast Connect.

FCC
Federal Communications Commission. An independent US government agency charged with regulating interstate and international communications by radio, television, satellite and cable.

FCIF
Full Common Intermediate Format or Full CIF. Used when distinguishing between CIF and QCIF. See CIF and QCIF.

FECC
Far-End Camera Control.

Fiber Optics
A communications medium utilizing laser or "light" transmission. Uses a glass or plastic fiber carrying light to transmit voice, data and video signals. Each fiber can carry from 90 to 150Mb/s.

Field
One half of a video frame. One field contains all of the odd or all of the even scanning lines of the picture.

Firewall
A barrier device placed between two separate networks. A firewall can be implemented in a single router that filters out unwanted packets or it can use a variety of technologies in a combination of routers and hosts. Today many firewalls combine filtering functionality with Network Address Translations (NAT) functions.

Flow Control
Comprised of the hardware, software and procedure for controlling the transfer of IP packets between two points on a network.

Foot-candle
The unit of illumination equal to one lumen per square foot.

Four-wire Circuit
A circuit that uses two pair of conductors (four wires), one pair to transmit the signals and one pair to receive; allows two parties to communicate simultaneously.

Fps
Frames per second (video).

Frame
Full screen or frame of video made up of two fields. Thirty frames is one second of video.

Frame Rate
The rate at which individual pictures (frames) in a sequence are displayed in a video conference. Frame rate is measured in frames per second (fps).

Full-duplex audio
Two-way audio may be captured and reproduced simultaneously. With full-duplex audio, the microphone may capture local audio for transmission while the far end audio may be heard clearly. Interruptions and double-talk are possible.

Full-motion Video
Not compressed. A standard video signal of thirty frames per second (fps), 525 horizontal lines per frame, capable of complete action.

Gatekeeper
In H.323 IP telephony, a Gatekeeper is an optional server that is responsible for network-based services including a registration, admission, and status, for which it uses a special protocol called RAS. Gatekeeper functions include address translation, call authorization and bandwidth management, as well as providing accounting information.

Gatekeeper Zone
A group of endpoints together with their Gatekeeper constitute a zone. You can configure a zone by predefining endpoints that are entitled to register with the Gatekeeper. See also predefined endpoint, registered endpoint, zone.

Gateway
A network element that performs conversions between different coding and transmission formats. The gateway does this by having many types of commonly used transmission equipment and / or circuits from different carriers to provide a means of interconnection. See Bridge.

Gateway Redundancy
The Gateway Redundancy feature enables the Gatekeeper to request that other Gatekeepers search their zones to locate a request when Line Hunting fails to find a suitable service provider. If no service is found, the Gateway Redundancy policy enables the Gatekeeper to complete the call by referring the call to other Gatekeepers.

Gateway Supported Prefixes
H.323 version 2 enables a Gateway to specify prefixes that the user should dial before the WAN number in order to make a call using a certain media. GCF:GATEKEEPER CONFIRM Message -A RAS message that the Gatekeeper sends to the requesting endpoint, accepting the GRQ.

GRJ
GATEKEEPER REJECT Message - A RAS message that the Gatekeeper sends to the requesting endpoint, rejecting the GRQ.

Group Hunting
Group Hunting enables a Gatekeeper to perform load balancing for a group of H.323 endpoints defined with the same alias.

GRQ
GATEKEEPER REQUEST Message - A RAS message that an endpoint sends to locate a Gatekeeper to which the endpoint can register.

GSM
Global System for Mobile Communications - The standard digital cellular phone service of Europe, Japan, Australia and elsewhere.

GOB
Groups of Blocks. In the encoding process, each picture is subdivided into groups of blocks and then further divided into macro blocks.

HMLP
High-speed multi-layer protocol. Standard for high-speed transmission. T.120 systems must use this for high-speed transfer. HMLP channels must be multiples of 64Kb/s.

H.221
A CCITT standard describing a method of inverse multiplexing for videoconferencing terminals, to be used with PX64 videoconferencing.

H.225.0
H.225.0 specifies that procedures and messages applicable to Gatekeepers, including the RAS protocol for Registration, Admission and Status.

H.235 Security
H.235 Security ensures the authentication of each endpoint and the integrity of messages. The basis of the security process is the shared secret between the endpoint and the Gatekeeper.

H.239
ITU-T standard for role management and additional media channels for H.300 - series terminals.

H.243
Standard for multipoint communication in H.320.

H.244
A CCITT standard that is an enhanced version of H.221. H.221's logical successor.

H.245
The ITU-T standard used for the Control Protocol for Multimedia Communication. H.245 is included in the H.225.0 Recommendation. H.245 provides signaling for the proper operation of the H.323 terminal, including capabilities exchange, opening and closing of logical channels together with a full description of these channels, mode preference requests, flow control messages, and general commands and indications.

H.245 Proxy
The H.245 Proxy enables routing H.245 channels in a point-to point H.323 call. The H.245 Proxy is a natural extension of a Gatekeeper.

H.245 Tunneling
H.245 tunneling decreases the time between the point at which and endpoint initiates a call and the point at which the call participants are ready to open multimedia channels. The endpoint must also support H.245 tunneling.

H.248
The ITU-T standard for signaling and control between circuit-switched PSTNs and VoIP networks.

H.261
A CCITT standard describing a protocol for digitally encoding and decoding video images to allow videoconferencing terminals from different manufacturers to interoperate.

H.263
Video compression standard created and required for H.324. Is also an optional mode of the other H-series standards.

H.264
The ITU-T standard for compression that allows higher quality calls to pass over a lower bandwidth for advanced video coding in generic audiovisual services.

H.320
A set of CCITT standards describing a method of interoperability between videoconferencing terminals from different manufacturers.

H.324/M
The Third Generation Partnership Project (3GPP) has adopted the ITU-T Recommendation H.324 Annex C, also referred to as H.324/M (Mobile) or H.324M (with very few changes), on their 3G-324M as the standard for multimedia telephony over 3G circuit switched cellular networks.

H.450
See Supplementary Services.

HDTV
Higher than normal definition TV. HDTV is generally defined as a system that offers double the horizontal and vertical resolution compared to existing systems and provides compact disc quality sound.

IETF
Internet Engineering Task Force. Formed in 1986, the IETF sets the technical standards that run the Internet. IETF working groups seek the advice of the Internet community through RFCs (requests for Comment), and then submit recommendations to the IETF for final approval.

IP Address
The unique address of a computer attached to a TCP/IP network. IP addresses are 32 bits long. Each octet is represented in decimal and is separated by dots.

IP Multicast
A means of simultaneous transmission of data from a server to a group of selected users on a TCP/IP network, (internal, intranet or Internet). IP multicast is used for streaming audio and video over the network.

IP Network
A network t hat uses the TCP/IP protocol.

IP Telephony
A set of technologies that enables voice, data and video collaboration over existing IP-based LANS, WANs, and the Internet. IP technology uses open IETF and ITU standards to move multimedia traffic over any network that uses IP.

IRQ
INFORMATION REQUEST Message. - A RAS message in which the Gatekeeper asks the endpoint for its current status.

IRR
INFORMATION REQUEST RESPONSE Message - A RAS message that the Gatekeeper sends to the calling endpoint, rejecting the IRQ.

ISDN
Integrated Services Digital Network. A set of standards that provide a common architecture for the development and deployment of digitally integrated communications services. A set of standardized customer interfaces and signaling protocols for delivering digital circuit-switched voice / data / video and packet-switched data services.

ISDN Rollover
In RADVISION implementations, Gateway support for the ISDN Rollover feature ensures that a call is completed even when call volume is high. ISDN Rollover requires support by the PSTN.

ITU
International Telecommunications Union. Organization composed of the telecommunications administrations of the participating nations. Focus is the maintenance and extension of international cooperation for improving telecommunications development and applications.

ITU-T
Standards body under the jurisdiction of the United Nations. Responsible for all international telecommunications standards.

IVDS
Interactive Video and Data Services. The name for the license which will be granted by the FCC for devices called Interactive TV Appliances (ITAs). ITAs include TV answer, a two-way television service for consumers for game shows, sporting events and respond instantly to new polls, interactive advertising as well as distance learning. The system will allow viewers to shop, bank, pay bills and order a pizza.

IVR
Interactive Voice Response is a two-stage incoming call routing method supported by the Gateway. It enables DID to a LAN terminal, even when the ISDN lines do not have multiple numbers allowing direct dialing to an endpoint.

IXC
Inter Exchange Carrier. Common carrier providing communications channels between local companies (LECs, or Local Exchange Carriers). Also known as long distance carriers such as AT&T, LCI, LDDS, MCI, US Sprint, WilTel, etc.

Jitter
The result of a change in latency or the tendency towards lack of synchronization caused by mechanical or electrical changes. Technically, jitter is the phase shift of digital pulses over a transmission medium.

Jitter Buffer
A portion of memory specifically allocated to storing IP packets awaiting transmission, or to storing received IP packets. The buffer facilitates flow control by capturing IP packets and then transmitting packets as "playback" using speeds and rates of delay that the destination device can handle without causing packet loss through overloading.

JPEG
Joint Photographic Expert Group. JPEG is an industry standard for still image compression that has moved into full motion video. JPEG is a compression technique based upon intraframe encoding technology. It allows for the full restoration of symmetrically compressed images.

Kbps
Kilobits per second. A unit of measure of data of 1,000 bits per second.

LAN
Local Area Network. A private transmission network interconnecting offices within a building or a group of buildings used to convey voice, data and video traffic.

LAN/WAN Connectivity
The practical set of tools, from operating system layer protocols to support services that make a remote access device an effective link between LANs and WANs.

Latency
A measure of accumulated waiting time or delay, representing the length of time required for information to pass through a network.

Lavaliere
A small microphone that can be clipped onto clothing or suspended from neck cords and worn in front of the chest.

LCF
LOCATION CONFIRM Message - A RAS message that a Gatekeeper sends to the requesting Gatekeeper/endpoint accepting the LRQ.

LDAP
Lightweight Directory Access Protocol - A protocol for accessing online directory services. LDAP is both an information model and a protocol for querying and manipulating the model.

Leased Lines
A term used to describe the leased or rented use of dedicated lines between two points.

LED
Light Emitting Diode. A display technology that uses a semiconductor diode that emits light when charged. LEDs usually indicate both correct and problematic operation.

Line Hunting
A Gateway supports a list of prefixes (services). Gateway unavailability to receive a call means that the Gateway cannot accept calls with the particular prefix in question. The Line Hunting function of a Gatekeeper searches for a Gateway which is free to accept calls with this prefix.

Local Loop
Referred to as the "last mile" between the LEC and the customer premise.

Load Balancing
The practice of splitting communication into two (or more) routes. By balancing the traffic on each route, communication is made faster and more reliable.

LRQ
LOCATION REQUEST Message - A RAS message that a Gatekeeper/endpoint sends to another Gatekeeper requesting the location of an endpoint.

LRJ
LOCATION REJECT Message - A RAS message that a Gatekeeper/endpoint sends to the requesting Gatekeeper rejecting the LRQ.

Lumen
Unit of light flux.

MAC
Media Access Control - A system of rules used to move data from one physical medium to another.

Master
The side in communications which initiates and controls the session. The "slave" is the other side that responds to the master's commands.

Mbps
Megabits per second. A unit of measure of data of 1,000,000 bits per second.

MGCP/MEGACO
Media Gateway Control Protocol/MEdia GAteway Controller - An IP telephony signaling protocol from the IETF. MGCP was the original protocol, which evolved into MEGACO. Both protocols are designed for implementation in IP phones that are cheaper than SIP or H.323 phones.

MLP
Multi-layer protocol for data (in H.221). MLP data and audio can only be placed in the first 64Kb/s channels of a connection. T.120 must use the MLP or HMLP channel.

MPEG
Motion Pictures Experts Group. Multimedia compression standard for professional and consumer applications such as digital video, digital audio and systems compression. MPEG compresses similar frames of video, tracks elements which change between frames and discards the redundant information.

MPEG-4
Moving Pictures Experts Group. MPEG is a series of standards designed to reduce the storage requirements of digital video. MPEG-4 provides the standardized technological elements for the integration of interactive graphics applications and interactive multimedia.

MSN
Multiple Subscriber Number - A method of incoming call routing in which a group of phone numbers is assigned to a particular ISDN line by the telephone company. PRI ISDN lines are usually assigned multiple numbers in the US and in Europe.

Multiplexing
The process of combining a number of individual channels into a common frequency band or into a common bit stream for transmission. The converse equipment or process for separating a multiplexed stream into individual channels is called a demultiplexer.

Multipoint
A call involving three or more parties.

Multipoint Videoconferencing
Videoconference with more than two sites. The sites must connect via a video bridge.

Multi-Unicast
Transmission of duplicate data streams, one to each used. In multi-unicast, multiple users request the same data from the same server at the same time. Contrast with IP multicast, unicast.

NAT
Network Address Translation - NAT devices translate IP addresses so that users on a private network can see the public network, but public network users cannot see the private network users.

Neighbor Gatekeeper
A mechanism by which the RADVISION H.323 Gatekeeper optimizes inter-zone communication. A list of Neighbor Gatekeepers and their IP addresses allows the Gatekeeper to resolve destination IP addresses when the source endpoint is not in the same zone as the destination endpoint.

Network
A group of stations (computers, telephones, or other devices) connected by communications facilities for exchanging information. Connection can be permanent, via cable, or temporary, through telephone or other communication links. The transmission medium can be physical (fiber optic cable) or wireless (satellite).

Network Load Balancing
See RAI/RAC

Non-Composite Video Signal
A signal which contains only the picture signal and the blanking pulses.

NT1
Network termination equipment for ISDN BRI. Turns a U interface into an S/T interface. Not needed when connecting to most digital PBXs.

NTSC
National Television System Committee. Defined the 525 line color video frequency spectrum used in the US, Canada, Mexico, Japan and a few other countries.

Online Endpoint
When an endpoint registers with a Gatekeeper, the endpoint is active and ready to receive calls. By registering, the endpoint informs the Gatekeeper that it is online.

Packet
A block of data used for transmission in packet-switched systems.

Packet Loss
The discarding of data packets in a network when a device is overloaded and cannot accept any incoming data at a given moment.

Packet Re-Ordering
Packet reordering ensures that all packets reach their destination in the correct sequential order.

Packet Switching
A network technology that breaks up a message into smaller packets for transmission and switches them to their required destination.

PAL
Phase Alternation by Line. The 625 line, 25 frame per second TV standard used in Western Europe, India, China, Argentina and parts of Africa. Brazil uses PAL-M, a 525 line variant.

Parent Filters
When the RADVISION Gatekeeper fails to resolve a destination address, the Gatekeeper searches for the destination first among its Children, then among its neighbors and then via its parent. Parent filters enable the Gatekeeper to avoid unnecessary searches among its Children and Neighbor Gatekeepers.

Party Number
The dialing number of an endpoint. This number can be a telephone number or a number used by other mechanisms on various networks, such as telex and ISDN.

PBX
Private Branch Exchange. A private telephone exchange that serves a particular organization or business and has connections to the public telephone network. Newer PBXs have features that allow for data and video communications as well as voice.

Picture Signal
That portion of the composite video signal which lies above the blanking level and contains picture brightness information.

PIP
Picture in Picture. In videoconferencing, the ability to view the near end (you) in a small, segmented portion of the monitor screen while viewing the far end (them) simultaneously in a larger segmented portion of the screen.

Pixel
The smallest controllable element that can be illuminated on a display screen. Related to resolution.

Point to Point
A videoconference between only two points.

Point to Multipoint
A videoconference between one location to many.

POP
Point of Presence. A central office where the inter-exchange carrier's responsibilities for the line begins and the local exchange carrier's responsibility ends. Location of a communications carrier's switching or terminal equipment. (Cincinnati to AT&T)

PORTL
A pathway into and out of a computer or a network device, such as a switch or a router.

POTS
Plain Old Telephone Service. Dial tone.

Predefined Endpoint
An endpoint entitled to register with a specified Gatekeeper.

Prefix
A prefix is a part of the dialing sequence used to access a service or conference type. See also Gateway supported prefixes and conferencing service.

PRI
Primary Rate Interface. An ISDN subscriber line, consisting of twenty three 64 Kb/s B bearer or user channels and one 64 Kb/s D channel, used for signaling and synchronization.

Projection Television
A combination of lenses and / or mirrors that project an enlarged video picture on a screen.

Protocol
A set of rules and procedures for establishing and controlling the transmission on a line. The set of messages has specific formats for exchanging communications and assuring end-to-end integrity of links, circuits, messages, sessions and application processes.

Proxy Server
An application that breaks the connection between sender and receiver. All input is forwarded out on a different port, closing a straight path between two networks and preventing a cracker from obtaining internal addresses and details of a private network.

PSDN
Public Switched Digital Network. A term used to describe the set of digital dial-up services offered by the carriers (IXC and LEC).

PSTN
Public Switched Telephone Network - The worldwide voice telephone network. Once only an analog system, the heart of most telephone networks today is all digital. In the US, most of the remaining analog lines are the ones from your house or office to the telephone company's central office.

Public Network
A network operated by the carriers (IXC and LEC) which includes network-based services and network-based switching.

Px64
A common reference to the CCITT standards (H.261 et al.) which describe methods to allow for videoconferencing system interoperability.

Q.931
A protocol for Call Signaling, consisting of Setup, Teardown and Disengage. Q.931 is included in the H.225.0 Recommendation.

Q.931 + H.245 Routed Mode
The routing of the Call Setup channel (Q.931) and the Control channel (H.245) through the Gatekeeper. See also Call Setup routing, Routed Mode.

QCIF
Quarter Common Intermediate Format. The QCIF format employs half the spatial resolution of CIF (both horizontal and vertical) and is the mandatory H.261 format. During encoding, a QCIF picture is subdivided into 3 GOBs (Groups of Blocks) Versus CIFs 12 GOBs.

QOS
Quality of Service - The ability to define a level of performance in a data communications system. For example, the ATM networks specify modes of service that ensure optimum performance for traffic such as real-time voice and video.

Radius
Remote Access Dial-In User Service - A server for authentication, authorization and accounting of endpoints and endpoint aliases.

RAI/RAC
Resource Available Indication / Resource Available Confirmation - The RAI/RAC function automatically manages load balancing on the network. RAI/RAC messages are exchanged between a Gatekeeper and a Gateway to determine whether the Gateway is available to receive calls.

RAS
A protocol for Registration, Admission and Status. In an H.322 audio or video system, the RAS is a control channel over which H.225.0 signaling messages are sent.

Raster
The scanned (illuminated) area of a television picture tube.

RBOC
Regional Bell Operated Company. The name given to the seven telephone companies created subsequent to the break-up of AT &T. Often, RBOC's own the local exchange carrier (LEC). For instance, Ameritech (RBOC) owns Ohio Bell (LEC).

RCF
REGISTRATION CONFIRM Message. A RAS message that a Gatekeeper sends to the calling endpoint accepting the RRQ.

Real-Time
The processing of information that returns a result so rapidly that the interaction appears to be instantaneous. Videoconferencing is an example of a real-time application.

Real-Time Streaming
Delivery of a real-time stream of a live videoconference while the conference is in progress.

Redundancy
See Gateway Redundancy

Registered Endpoint
A registered endpoint is an endpoint that has informed the Gatekeeper that it is online, active and ready to receive calls, and has received confirmation from the Gatekeeper of its registration request.

RFP
Request for proposal. A bid that specifies and describes a system in industry terminology which the vendors understand. An RFP will prompt vendors to respond to questions about installation, training, maintenance, warranty, purchase terms and other relevant issues.

RGB
Method of transmitting video signals that feeds red, green and blue channels over separate wires; provides the highest quality video signal and is the format for most computer equipment.

Routed Mode
The routing of the Call Setup channel (Q.931) and the Control channel (H.245) through the Gatekeeper. See also Call Setup routing, Q.931 + H.245 Routed Mode.

Router
A device or setup that finds the best route between any two networks, even if there are several networks to traverse. Like bridges, remote sites cab be connected using routers over dedicated or switched lines to create WANs.

RRJ
REGISTRATION REJECT message. A RAS message that a Gatekeeper sends to the requesting endpoint rejecting the RPQ.

RRQ
REGISTRATION REQUEST. A RAS message in which an endpoint identifies itself to a specific Gatekeeper and asks for service. The RRQ message binds the endpoint aliases-names or phone numbers- to the IP addresses of the endpoint.

RS366
A standard for providing dialing commands to network access equipment. In a videoconferencing application, an RS366 links the video codec and the network access equipment in order to facilitate dialing from the video codec. (e.g., IMUX)

RTP/RTCP
Real Time Transport Protocol / Real Time Control Protocol-RTP is an IP protocol that supports real-time transmission of voice and video. It is widely used for IP telephony. RTCP is a companion protocol that is used to maintain QoS.

RTP Redundancy
A method of overcoming packet loss by doubling packet payload without increasing the number of packets sent.

SDSAF
Switched Digital Services Applications Forum. A consortium of equipment vendors, service providers and users, with the goal of advancing the state of switched digital services.

SECAM
System Electronique Pour Couleur Avec Memoire. The 625 line, 25 frame per second color television system used in France, Eastern Europe, USSR and parts of Africa.

Serial Interface
A channel that transfers digital data in a serial fashion, one bit after the other over one wire or fiber. The serial port on a PC is a serial interface that is used to attach modems and scanners. Serial interfaces may have multiple lines, but only one is used for data. See also V.35 and x.21.

Service Prefix
The prefix identifies the service and can usually be a numeric code, an alphanumeric string, a name, or phone number.

Services
A service is a function that is supported by a subset of endpoints in a zone. Access a service by dialing a prefix attached to the name or phone number. Services allow you to dynamically add more resources, such as a Gateway, into the system. In RADVISION implementations you can define access privileges per endpoint for each service.

Silence Suppression
Silence information within the audio stream can consume LAN bandwidth and burden MCU voice processing. Using compression techniques, Silence Suppression can greatly reduce the wasted bandwidth in a multipoint conference and on congested networks.

Single Station MCU
In RADVISION implementations, a solitary RADVISION MCU that is facilitating a conference.

SIP
Session Initiation Protocol - An IP telephony signaling protocol developed by the IETF. SIP is a text-based protocol that is suitable for integrated voice-data applications. SIP is designed for voice transmission and uses fewer resources and is considerably less complex than H.323.

Slave
The side in communications which responds to session commands. The "master" is the other side that initiates and controls the session.

S/NR
Signal to Noise Ratio. Final relationship between the video or audio signal level to the noise level. Ratio of the signal power to the noise power in a specified bandwidth, expressed in dbW.

SQCIF
Sub Quarter Common Intermediate Format. A video format usually used to refer to video with dimensions of 88 x 72 pixels.

Still-Image Video
System by which still images are transmitted over standard telephone lines, usually allowing for real-time interaction between locations.

Sub-Addressing
Sub-addressing is a one-stage DID dialing mechanism in which a phone sends two numbers. One number is for routing on the circuit-switched network. The other number is forwarded to the Gateway inside a Q.931 Sub-addressing Information Element for IP address resolution by the Gatekeeper.

Sub-Conference
The current version of the RADVISION MCU allows the creation of audio sub-conferences to which selected participants in an existing conference can be temporarily diverted. When the sub-conference is over, the participants return to the main conference.

Subnet
A subnet is a portion of an IP network defined by a subnet mask. Devices on the same subnet have the same subnet mask.

Supplementary Services
The ITU-T H.450 Recommendation comprises a number of separate Recommendations for implementing Supplementary Services, of the H.450.x series. For full details of the H.450.x Recommendations, please refer to the appropriate ITU documents.

Switch
A mechanical or solid state device that opens and closes circuits, changes operating parameters or selects paths for circuits on a space or time division basis.

Switched 56
A dial-up network-based service providing a data channel operating at a rate of 56Kb/s. Also a type of network access line, used to provided access to switched 56 network services.

Switched 64
A dial-up network-based service providing a data channel operating at a rate of 64Kb/s.

Switched 384
A dial-up network-based service providing a data channel operating at a rate of 384Kb/s.

Switched 1536
A dial-up network-based service providing a data channel operating at a rate of 1536Kb/s.

Switched Network
Any network in which switching is present and is used to direct messages from the sender to the recipient. Usually, switching is accomplished by disconnecting and reconnecting lines in different configurations in order to set up a continuous pathway between the sender and the recipient.

T1
A digital transmission link with the capacity of 1.544Mb/s, used in North America. Typically channelized in 24 DSO's, each capable of carrying a single voice conversation or data stream. Uses two pairs of twisted pair wires.

T.128
Formally called "T-share", used in multi-point data conferencing.

T3
A digital transmission link with a capacity of 45Mb/s, or 28 T1 lines.

Tariff
Documents filed by a regulated telephone company with a state public utility commission (PUC) or the Federal Communications Commission (FCC). Document details services, equipment and pricing publicly offered by the telephone company.

TCP/IP
Transmission Control Protocol / Internet Protocol. Transmission Control Protocol/Internet Protocol. A set of protocols developed by the Department of Defense to link dissimilar computers across many kinds of networks, including unreliable ones.

TCS4
TCS4 is a special routing method for incoming H.320 video calls. TCS4 allows direct inward dialing to an endpoint on the IP network via the Gateway when DID is not available.

Telco
Generic name for telephone companies.

Telecommunications
Communicating over a distance. Use of wire, radio, optical or other electromagnetic channels to transmit and receive signals for voice, data and video communications.

Topology Islands
IP subnets, characterized by homogeneous and fast LAN connectivity. Dividing the network into islands enables a Gatekeeper to direct calls through the most optimal routes, thus avoiding slow connections or bottlenecks as much as possible.

TPKT
A standard way of defining blocks of data in a TCP stream, since TCP does not have delimiters. During configuration you can define the maximum number of TPKT channels allowed.

Transcoding
Audio transcoding is the conversion of one audio transmission format into another using various algorithms to achieve different audio quality levels at reduced bandwidth levels.

TTL
Time to Live. A set maximum amount of time a packet is allowed to propagate through the network before it is discarded. TTL is a time, typically in seconds, after which the fragment can be deleted by any device on the network.

Twisted Pair
A pair of wires used in transmission circuits and twisted about one another to minimize coupling with other circuits.

UCF
UNREGISTRATION CONFRIM Message - A RAS message that a Gatekeeper or an endpoint sends accepting the URQ.

UDP
User Datagram Protocol - A transport protocol within the TCP/IP protocol suite that is used in place of TCP when a reliable delivery is not required.

Unicast
A means of transmitting a message from one station to another; contrast with IP.

Unrecognized Alias
An alias that is not in the registration database of the Gatekeeper.

Unregistered Endpoint
An endpoint that is no longer online and registered with a Gatekeeper.

URJ
UNREGISTRATION REJECT message. A RAS message that a Gatekeeper or an endpoint sends rejecting the URQ.

URL
Uniform Resource Locator - An Internet address. The address that defines the route to a file on a computer connected to the Internet.

URQ
UNREGISTRATION REQUEST Message. - A RAS message sent when an endpoint wishes to terminate its session with a Gatekeeper.

V.35
An ITU standard for group band modems that achieve higher data rates by combining the bandwidth of several telephone circuits. Typically used for modem-to-multiplexer communication. See also serial interface and X.21

VCS
Video Conferencing System.

Video Bit Rate
Bit rate is the speed at which bits are transmitted, in bits per second.

Video on Demand Streaming
Delivery of a Video on Demand stream to a viewer upon request at any given time. Contrast this to a real-time stream that is delivered when the conference is in progress.

Voice-Activated Video Switching
Automatic switching of a video image viewed at each conference terminal according to the voice level of a each participant.

Voice Switched Video
Type of video conference in which the cameras are activated by voice signals to send a picture of a particular person in the group. Not all participants are seen at any one time in contrast to continuous presence video.

VOIP
Voice over Internet Protocol (VoIP) is a protocol optimized for the transmission of voice through the Internet or other packet switched networks. VoIP is often used abstractly to refer to the actual transmission of voice (rather than the protocol implementing it). VoIP is also known as IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband. "VoIP" is pronounced voyp.

VPN
Virtual Private Network - VPN modules create closed secure tunnels for communication between two firewalled LANs. VPN technology is one of the approached being used today for providing secure communications over IP networks.

WAN
Wide Area Network. A data network typically extending a LAN outside a building or beyond a campus, over IXC or LEC lines to link other LAN's at remote sites. Typically created by using bridges or routers to connect geographically separated LANs.

Web Conferencing
Enables two or more logged in users to set up a typed, real-time, online conversation across the World Wide Web.

Whiteboard
A term used to describe the placement of shared documents on an on-screen "shared notebook". See also document sharing.

Wildcard Digit Manipulation
A mechanism which enables a Gatekeeper to manipulate an incoming call source number before searching for the destination endpoint.

X.21
An ITU standard protocol for a circuit switched network. See also serial interface and V.35.

Zone
An H.323 zone is a logical collection of terminals, Gateways and Multipoint Control Units (MCUs) managed by a single Gatekeeper. A zone must include at least one terminal and may include several LAN segments connected by routers. See also Gatekeeper Zone.

Zone Prefix
Zone prefixes are similar to telephone are codes. If an endpoint in a zone dials a zone prefix before its number, and the Gatekeeper cannot resolve it in its zone, the Gatekeeper attempts to locate and route the call to the appropriate zone of the Neighbor Gatekeeper.